A wire-discharge machining apparatus applies a high-frequency pulse voltage to a wire electrode and removes the workpiece little by little using an electric discharge caused between the work electrode and the workpiece by the application of the voltage to machine the workpiece into a predetermined shape. The wire electrode travels in a predetermined direction, for example, a vertical direction while being guided by a pair of upper and lower wire guides in the direction. Machining fluid is supplied to an area around the wire electrode while the workpiece is machined. For example, it is possible to accurately machine the workpiece by causing the electric discharge while moving a table having the workpiece placed thereon in the predetermined direction according to numerical control.
In machining the workpiece accurately and stably with the wire electric discharge machining, it is important to machine the workpiece while keeping an interval between the wire electrode and the workpiece within a predetermined range. When the wire electrode and the workpiece are short-circuited, the machining stops because the electric discharge is not caused. Concentration of electric discharges (hereinafter, “concentrated electric discharge”) occurs because, for example, machining wastes accumulates between the wire electrode and the workpiece and electric discharge energy excessively increases locally. As a result, breakage of the wire electrode (hereinafter, “wire-breakage”) often occurs. When the wire-breakage occurs, since the wire electrode has to be guided by the wire guides again, productivity substantially falls. Thus, various technologies for preventing the wire-breakage have been devised.
For example, a Patent Document 1 discloses an electric discharge machining apparatus in which two or more electricity conducting terminals for supplying a pulse voltage from a machining power supply to a wire electrode are provided above and below a workpiece, electricity conduction switches are provided between the respective electricity conducting terminals and the machining power supply, and the electricity conduction switches are controlled to be changed over every time a plurality of continuous pulse voltages are applied from the machining power supply to one electricity conducting terminal. In this electric discharge machining apparatus, since an electric discharge point between the wire electrode and the workpiece periodically moves up and down, heat generation from the wire electrode is controlled even if a large current is applied. Moreover, since electric discharge points between the wire electrode and the workpiece are dispersed, wire-breakage is prevented.
A Patent Document 2 discloses a wire-cut electric discharge machining apparatus in which electricity conducting elements for supplying a machining pulse to a wire electrode are provided on an upper-side and a lower-side of a workpiece and machining pulse power supplies are provided separately between the upper electricity conducting element and the workpiece and between the lower electricity conducting element and the workpiece, respectively. In this wire-cut electric discharge machining apparatus, pulse electric currents are asynchronously fed to the wire electrode from the upper electricity conducting element and the lower electricity conducting element, respectively, to prevent concentration of electric discharge points and, as a result, prevent wire-breakage.
A Patent Document 3 discloses an electric discharge machining apparatus in which two contacts are provided along a wire electrode to be located at both ends of a machining area in a workpiece and a machining electric current is supplied to one or both of the two contacts depending on an electric discharge position between the wire electrode and the workpiece. In this electric discharge machining apparatus, the contact to which the machining electric current should be supplied is changed depending on the electric discharge position to prevent local heating due to concentrated electric discharge and, as a result, prevent wire-breakage.
Patent Document 1: Japanese Patent Application Laid-Open No. S59-47123
Patent Document 2: Japanese Patent Application Laid-Open No. H1-97525
Patent Document 3: Japanese Patent Publication No. H6-61663